1. Field of the Invention
The present invention relates to a solid state image sensor, and more particularly, to a solid state image sensor and a method for fabricating the same, which can improve a sensitivity and a smear of a CCD(Charge Coupled Device).
2. Background of the Invention
In general, a solid state image sensor is a device which uses a photoelectric conversion device and a charge coupled device in taking an image of an objective and providing the image as an electrical signal. The charge coupled device is used in transmission of a signal charge generated in a photoelectric conversion device(a photodiode) through a microlens and a color filter in a particular direction in a substrate utilizing a potential variation. The solid state image sensor is provided with a plurality of photoelectric conversion regions, vertical charge coupled devices(VCCDs) each one formed between the photoelectric conversion regions for vertical transmission of charges generated in the photoelectric conversion regions, a horizontal charge coupled device(HCCD) for horizontal transmission of the charge transmitted in the vertical direction by the VCCDs, and a floating diffusion region for sensing and amplifying the charges transmitted in the horizontal direction and providing to a peripheral circuit.
A related art method for fabricating an HCCD will be explained with reference to the attached drawings. FIGS. 1a.about.1d illustrate sections showing the steps of a related art method for fabricating a solid state image sensor.
Referring to FIG. 1a, the related art method for fabricating a solid state image sensor starts from forming a first planar layer 14 on a black and white solid state image sensor 11 which is provided with a plurality of photodiode regions PD 12 each for converting an optical image signal into an electrical signal, a plurality of vertical charge coupled devices(VCCDs) 13 each for transmitting video charges generated in the photodiode region 12 in a vertical direction, and a horizontal charge coupled device(not shown) for transmission of the video charges transmitted in the vertical direction to a horizontal direction. Then, as shown in FIG. 1b, first, second and third color filter layers 15, 16 and 17 are formed on the first planar layer 14 over the photodiode regions 12 in succession to correspond to the photodiodes 12, selectively. The color filter layers are formed by coating and patterning a dyeable resist and dying and fixing the dyeable resist layer. As shown in FIG. 1c, a second planar layer 18 is formed on an entire surface inclusive of the first, second and third color filter layers 15, 16 and 17. Then, as shown in FIG. 1d, microlenses 19 are formed on the second planar layer 18 to correspond to respective photodiode regions 12.
The operation of the related art solid state image sensor formed by the aforementioned method will be explained.
A light incident to the solid state image sensor is focused by a microlens 19, passes through one of the first, second and third color filter layers 15, 16 and 17 each of which is adapted to transmit only a particular waveform of light, and is incident to a corresponding diode 12. The light incident to the photodiode 12 is converted into an electrical signal at the photodiode region 12, and transmitted in a vertical direction and a horizontal direction in response to clock signals applied to gates on the VCCDs and the HCCD of the solid state image sensor, sensed and amplified at the floating diffusion region(not shown) at an end of the HCCD, and provided to a peripheral circuit.
FIG. 2 illustrates focal distances of a short axis and a long axis of a related art microlens.
Referring to FIG. 2, the rectangular microlens formed in conformity to the rectangular pixel in the CCD, with a difference of radii of curvatures of the long axis and the short axis, forms different focal distances of the lights incident to the photodiode through the microlens. That is, a focal distance differs depending on refractive indices of the air and a lens, and since refractive indices of the air are fixed at the same place, focal distances of the lights transmitted through the microlens to be incident to the photodiode become different due to a difference of refractive indices coming from a difference of radii of curvatures; a focal distance from the long axis is greater than a focal distance from the short axis.
However, the related art method for fabricating a solid state image sensor has the following problem.
That is, the rectangular microlens formed in conformity to the rectangular pixel in the CCD causes a loss of CCD sensitivity as much as a difference of focal distances of the lights incident to the photodiode through the microlens.